The present application is the national stage under 35 U.S.C. 371 of PCT/IT98/00046, filed Mar. 4, 1998.
1. Field of the Invention
The present invention relates to a method for treating waste material contaminated with heavy metals so as to make the waste material. The treatment involves the use of calcium hydroxide and/or calcium oxide and a concentrated aqueous solution of orthophosphoric acid and a salt thereof with an alkaline or alkaline earth metal.
2. Background of the Invention
As is known, special and/or toxic harmful waste material from various sources containing heavy metals, such as arsenic, cadmium, chromium, mercury, lead, selenium and tellurium, which is liable to produce in transfer tests an eluate that does not comply with the established acceptability limits, needs to be disposed of in secured landfills for hazardous toxic wastes.
The prior art in this specific area records many techniques which allow metals to be immobilized in stable matrices, thereby making them chemically inert and thus preventing their leaching and of thereby allowing these types of waste material to be disposed under less severe conditions (leaching tests within limits imposed by national and international legislation).
Very often, these types of waste material have a composition which requires, for sufficient stabilization, the use of large amount of reagents, such as for example cement, or the use of expensive products, accompanied by long maturation times. Other solidification techniques make use of additives, such as slag, triturated bricks, pebble gravel, and sand mixed with bentonite, which results in excessive increases in the final weight and volume, accompanied by modest results in limiting the leaching o the metals.
In order to lower the binder component for example in the case of ash from mining flue gases, the interfering ions (chlorine ions) are washed with water.
A number of systems recently applied use aqueous solutions of chemical products that are particularly expensive, as a result of which not enough is known about the physical and physicochemical conditions required for the various types. of waste material, or about the amounts to be used in order to ensure the desired results.
In U.S. Pat. No. 4,737,356, the immobilization of lead and cadmium contained in solid residues takes place by the addition of water soluble. phosphate source. According to a specific embodiment, a mixture of Na5P3O10 and 85% H3PO4 (1:1) is used.
There is thus a need, in this specific field, to have available a versatile method which does not require particular pre-treatments and which uses readily available and low-cost reagents, to be added in modest amounts in order-to minimize the variations of the final weight.
The present invention makes it possible to satisfy the abovementioned requirements by also offering other advantages which will become apparent hereinbelow.
The subject of the present invention is thus a method for treating waste material contaminated with heavy metals so as to make the waste material inert by adding a water soluble phosphate source, wherein the material to be made inert, which is optionally at least in part neutralized, is subjected to the following operations, optionally under stirring:
addition of a concentrated aqueous solution of orthophosphoric acid and of an alkaline or an alkaline earth salt thereof in a molar ratio ranging from 1:5 to 5:1;
optionally, addition of water to give the homogeneous paste thus obtained a sufficient moisture content, the resulting homogeneous paste being placed in the open air to help it dry out;
addition of calcium hydroxide and/or calcium oxide.